The present invention relates to a contact sheet used with a socket employed for connecting an integrated circuit having spherical terminals arranged in a grid pattern, and a board, and used especially with a thin socket employed for high-frequency testing or mounting.
In recent years, because of the demands for reduced sizes and higher speed in information processing equipment, the pitches in integrated circuits are increasingly becoming smaller, and the mounting method is shifting from through-hole to surface mounting, and the arrays of terminals are shifting from peripheral arrays to grid arrays. As a result, ball grid array (BGA) devices are becoming the mainstream of packages as they permit surface mounting even for grid arrays.
A socket is used between a BGA type integrated circuit device and a board when conducting a burn-in test or a high-frequency test of the integrated circuit, and the socket is also mounted on a board when replacing an integrated, circuit. In a socket, all contacts must be in secure contact with both terminals of an integrated circuit and terminals of a board in order to ensure sufficient electrical conduction.
On the other hand, in recent years, with the increasing speed of MPUs or memories, the demand for lower inductance with respect to faster clocks has been arising. To meet the demand, it is necessary to minimize the distance over which currents pass in a contact placed between a terminal of an integrated circuit and a terminal of a board.
As a thin connector or socket with a small pitch, a component called xe2x80x9can anisotropic conductive sheetxe2x80x9d has conventionally been used. There have been known, for example, conductive sheets in which conductive elastomers or, metal wires are arranged in an insulative elastomer (U.S. Pat. No. 3,862,790 and U.S. Pat. No. 4,295,700) and one in which conductive particles are added (Japanese Unexamined Patent Publication No. 6-82521). FIG. 12 shows a conventional example, wherein thin metal wires 31 are embedded in a silicone rubber 30 at a high density.
Furthermore, as conductive sheets to which considerations have been given to the aspects of BGAs that their spherical terminals are soft and their surfaces are made of solder coated with oxide films, there is one with dendrites grown on contact portions (U.S. Pat. No. 5,691,041), one in which cantilevers having protrusions are fixed on a flexible board (U.S. Pat. No. 5,629,837), one in which the surfaces of terminals on a flexible board have been provided with asperities by chemical processing or the like (Japanese Unexamined Patent Publication No. 10-32070), and one in which innumerable protrusions are formed by thermal spraying or the like (Japanese Unexamined Patent Publication No. 10-144440), etc.
Furthermore, as ones with a structure for avoiding contact with ball tips while destroying oxide films on solder surfaces, there is one having Y-shaped contacts (Japanese Unexamined Patent Publication No. 9-21847), and one inserted in finger springs (U.S. Pat. Nos. 5,702,255 and 5,730,606).
As the number of terminals of an integrated circuit increases, the force required for insertion increases. There is a contact sheet having a structure in which the foregoing force is reduced to zero, and spherical terminals are pushed laterally later by using a lever or the like (U.S. Pat. Nos. 5,578,870 and 5,637,008). As this type, one employing vertically long cantilevers is the mainstream. Moreover, there is a socket described in Japanese Unexamined Patent Publication No. 9-55273 that permits replacement of contact portions while solving the problem peculiar to solder balls mentioned above.
However, in an anisotropic conductive sheet employing the elastomer as shown in FIG. 12, mostly, terminals having flat surfaces provided with gold plating are connected. If this conductive sheet is used for connection with a BGA having spherical terminals made of solder, solder surfaces will inevitably be dented, posing a problem of unstable electrical contact.
Furthermore, in those with asperities or protrusions formed on the surfaces of the terminals on the flexible boards, variations in height of terminals will cause insufficient electrical conduction in some circuits. In the Y-shaped contacts, the distance over which currents pass is relatively long, making them unsuited as contacts for high-frequency testing or mounting although reliable electrical conduction can be ensured.
The present invention has been made with a view toward solving the problems, and an object thereof is to provide a contact sheet that (1) enables reliable electrical contact to be obtained by removal of an oxide film from a solder surface, (2) is capable of accommodating variations in height or size of spherical terminals, (3) prevents collapse of a tip, (4) features a short distance over which currents pass, (5) permits a zero insertion force (ZIF) structure to be implemented as necessary, and (6) also enables a structure for permitting replacement to be implemented.
More specifically, according to the present invention, a contact sheet is provided to connect an electrical device (i.e., integrated circuit) having terminals and a board, having protruding contact springs in some or all of numerous through holes provided in a sheet composed of an insulative elastic material, wherein the contact springs are composed of a conductive material and constituted by two or more cantilevers fixed onto the sheet by bonding or clamping one end thereof, each cantilever has a lower supporting portion and a spherical terminal holding portion, the lower supporting portion is composed of a linear portion or a planar portion formed adjacently to a portion fixed to the sheet, the spherical terminal holding portion is formed by vertically bending the other end of the cantilever toward one of two openings of a through hole, the spherical terminal enters into a gap at the spherical terminal holding portion of the two or more cantilevers when the integrated circuit is mounted, and thereby the spherical terminal holding portions of the two or more cantilevers are pushed and spread in a direction away from the portion of each cantilever that is fixed to the sheet, and at the same time, a boundary portion between the spherical terminal holding portions and the lower supporting portion is pressed against the board.
Preferably, in the contact sheet set forth above, each contact spring is composed of three cantilevers, the three cantilevers are disposed virtually parallel to each other in the through hole, and two cantilevers on both ends of the three cantilevers and the middle cantilever are fixed to edges of the through hole that oppose each other because contact pressure on spherical terminals is well-balanced between a direction along the line and the direction perpendicular to the line, which makes spotting possible.
Furthermore, according to the present invention, there is provided a contact sheet used for a socket for achieving connection between an integrated circuit having spherical terminals and a board, having protruding contact springs in some or all of numerous through holes provided in a sheet composed of an insulative elastic material, wherein each of the contact springs is constituted by two or more cantilevers composed of a conductive material, the two or more cantilevers are respectively fixed to adjoining edges of the through hole at one end thereof so that each of them has two end portions and they are oriented in parallel to each other, each cantilever has a lower supporting portion and a spherical terminal holding portion, the lower supporting portion is composed of a linear portion or a planar portion formed adjacently to a portion fixed to the sheet, the spherical terminal holding portions are formed by vertically or obliquely bending the other ends of the two or more cantilevers toward one of two openings of a through hole, and when the integrated circuit is mounted, the spherical terminals push and spread the spherical terminal holding portions of the two or more cantilevers in a direction away from the portion of each cantilever that is fixed to the sheet and a boundary portion between the spherical terminal holding portions and the lower supporting portion is pressed against the board.
It is preferable that dispositions of two or more cantilevers installed in a through hole of an arbitrary one column and dispositions of the two cantilevers installed in a through hole of another column adjacent to the one column have a point symmetrical relationship because contact pressure on spherical terminals in a direction of a line is well-balanced by a pair of holes with the adjacent line, which makes spotting possible.
It is preferable in a contact sheet of the present invention that a through hole has a shape of a rectangle and a longitudinal direction of the rectangle coincides with a diagonal direction with reference to the disposition of the spherical terminals because the cantilevers can be made long, much displacement can be taken, and permanent set in fatigue as a spring is small.
Preferably, in the contact sheet in accordance with the present invention, it is preferable that each contact spring is composed of two or more cantilevers, the two or more cantilevers are disposed parallel to one another in the through hole, and a bridge connecting the lower supporting portion of the two or more cantilevers fixed to edges of the through holes adjacent to each other among the two or more cantilevers is provided because it avoids more than necessary separation of two spherical terminal holding portions when spherical terminals enters into the spherical terminal holding portions.
Further, as shown in FIG. 3(a), a cantilever of a contact sheet is preferably in contact with the board before an integrated circuit is mounted because the securer electrical conduction can be ensured.
In the present invention, it is preferable that two contact sheets are unitarily formed by means of an adhesive sheet having through holes of the same pattern as the contact sheets, and each of the contact sheets has the same number of contact springs on both sides of the adhesive sheet.
In addition, cantilevers of the contact springs provided on both side of the adhesive sheet are mutually in contact with one another with mutually applying pressure before the integrated circuit is mounted. For such an adhesive sheet, an anisotropic conductive membrane may be used.
Furthermore, in the contact sheet in accordance with the present invention, it is preferable that two sheets composed of an insulative elastic material are superposed and that one end of the cantilever is bonded by the two sheets so as to fix the cantilever to the sheet.